Nowadays, a three-way catalyst is widely used as a catalyst for purifying NOx (nitrogen oxides), CO carbon monoxide and HC (hydrocarbons) in exhaust gases from automobiles or the like. The three-way catalyst comprises a porous oxide supports such as alumina, ceria, zirconia and a ceria-zirconia solid solution, and a noble metal such as platinum (Pt), rhodium (Rh) and palladium (Pd) loaded on the supports, and simultaneously purifies HC and CO by oxidation and NOx by reduction. Since these reactions proceed most efficiently in an atmosphere including approximately equivalent amounts of oxidizing components and reducing components, in an automobile to which a three-way catalyst is installed, the air/fuel ratio is controlled so that an air-fuel mixture is burned at or near the theoretical air/fuel ratio, i.e., the stoichiometric point (A/F=about 14.6±0.2).
The three-way catalyst, however, has a problem that when the exhaust gas atmosphere becomes reductive, sulfur oxides in the exhaust gases are reduced into and emitted as H2S. Japanese Examined Patent Publication (KOUKOKU) No. H08-015,554 discloses the use of nickel or copper oxides as components of a three-way catalyst. The nickel or copper oxides oxidize SO2 into SO3 or SO4 in an oxidizing atmosphere and store sulfur components as sulfides such as Ni2S3 in a reducing atmosphere, whereby H2S emissions can be suppressed.
However, since nickel or copper is an environmentally hazardous substance, the use of nitrogen or copper in a catalyst for purifying exhaust gases from automobiles is getting inhibited. Therefore, consideration is given to the use of bismuth oxides, which exhibit a similar effect of suppressing H2S emissions to those of nickel or copper oxides. A Bi-containing catalyst, however, has a problem that bismuth components scatter at elevated temperatures and the effect of suppressing H2S emissions is not retained for a long time.
By the way, Japanese Examined Patent Publications (KOUKOKU) Nos. H02-020,561, H05-081,521, and S61-020,342 disclose Bi-containing catalysts. These catalysts, however, aim to oxidize H2S in oxidizing atmospheres. Therefore, when the air/fuel ratio is controlled as in the cases of automobiles, there is a fear that H2S is emitted in stoichiometric or reducing atmospheres. These patent documents do not refer to the catalytic effects in the stoichiometric or reducing atmospheres and do not disclose or suggest the use of these catalysts as three-way catalysts, which are also used in the stoichiometric or reducing atmospheres.